1. Field of the Invention
This invention relates to a 2,6-di-tert-butyl-4-methyl thiopyrylium salt that can be used as an intermediate for the synthesis of a new type of thiopyrylium dyes.
2. Description of the Prior Art
Thiopyrylium and pyrylium dyes are used for various purposes. For example, as disclosed in Japanese Patent Publication No. 40900/71, they are used in a direct positive photographic silver halide emulsion as an electron acceptor. They are also useful as a spectral sensitizer for a photoconductor, especially an organic photoconductor, as taught in Davis et al., U.S. Pat. No. 3,141,700, Van Allan et al., U.S. Pat. No. 3,250,615, and Reynolds et al., U.S. Pat. No. 3,938,994.
Photoconductors sensitized with thiopyrylium and pyrylium dyes are used in the various applications disclosed in the above mentioned patents, and they are particularly important for use in electrophotography such as xerography or electrofax.
The thiopyrylium dyes that can be synthesized from the compound of this invention are effectively used in not only conventional electrophotography such as xerography or electrofax but also photoelectrophoretic electrophotography. The dyes are particularly useful in color electrophotography using photoconductive particles. It is of course to be understood that the uses of the thiopyrylium dye are not limited to the ones mentioned above.
The principle of the photoelectrophoretic electrophotography is detailed in U.S. Pat. No. 3,384,488, according to which a suspension of electrically photosensitive particles in an insulating liquid is placed between a pair of electrodes, at least one of which is transparent to light and across which a differential voltage is applied, and the suspension is exposed imagewise through the transparent electrode. The electrically photosensitive particles selectively migrate to one electrode to form a visible image on that electrode.
To produce a polychromatic image, a suspension comprising a mixture of a cyan colored particle sensitive to red light, a magenta colored particle sensitive to green light and a yellow colored particle sensitive to blue light is set on the above-described system wherein it is exposed imagewise through a multicolor original image, e.g., a color slide, (or by the reflective printing method) using white light, and one operation of imagewise exposure produces a subtractive color positive image on the transparent electrode.
Illustrative particles suitable for producing such subtractive color images are described in U.S. Pat. No. 3,384,488, Japanese Patent Publication No. 21781/68 (U.S. Pat. Nos. 3,681,064 and 3,384,566) and Japanese Patent Application (OPI) No. 143827/77 (U.S. Pat. No. 4,032,339) (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), and they are cyan, magenta and yellow pigments the principal absorption bands of which correspond to their principal sensitive regions.
In addition to these three colored photoconductive pigments, U.S. Pat. No. 3,384,488 teaches electrically photosensitive particles which contain a spectral sensitizer so that they are sensitive to radiation in the visible spectral range. The spectral sensitizers must be sensitive selectively to red, green and blue radiation.
As Van Allan et al. and Reynolds et al. patents, supra teach, the thiopyrilium dyes have a high ability to sensitize photoconductors, especially organic photoconductors, but an attempt at performing color copying using the conventional thiopyrylium dyes as a sensitizer for the photoconductive particles has encountered a serious problem. The conventional thiopyrylium dyes that sensitize photoconductive materials for the light in the visible spectrum have a plurality of absorption bands within the visible spectrum, and most of them exhibit absorption to blue radiation. As a result, the sensitizing dyes cause spectral sensitization in the plurality of wavelength bands. Therefore, if the conventional thiopyrylium dye is used as a spectral sensitizer for photosensitive particles to produce a color image from a mixture of three-colored particles by the photoelectrophoretic electrophotography, the resulting image has insufficient color separation, demonstrating the inadequacy of the dye as a spectral sensitizer for use in the photoelectrophoretic electrophotography.
This problem is solved using a new type of thiopyrylium dye that is synthesized from the intermediate provided by this invention.